Ultra-durable superhydrophobic cellular coatings

Developing versatile, scalable, and durable coatings that resist the accretion of matters (liquid, vapor, and solid phases) in various operating environments is important to industrial applications, yet has proven challenging. Here, we report a cellular coating that imparts liquid-repellence, vapor-imperviousness, and solid-shedding capabilities without the need for complicated structures and fabrication processes. The key lies in designing basic cells consisting of rigid microshells and releasable nanoseeds, which together serve as a rigid shield and a bridge that chemically bonds with matrix and substrate. The durability and strong resistance to accretion of different matters of our cellular coating are evidenced by strong anti-abrasion, enhanced anti-corrosion against saltwater over 1000 h, and maintaining dry in complicated phase change conditions. The cells can be impregnated into diverse matrixes for facile mass production through scalable spraying. Our strategy provides a generic design blueprint for engineering ultra-durable coatings for a wide range of applications.

creative.This work provides a new feasible way for engineering ultra-durable coatings.However, some issues should be addressed after the review.
1.Although the authors demonstrated the excellent performance of the coating through experimental tests, the necessary theoretical analysis and discussion were missing for each performance test result.Such as these results of the tensile fracture strength, the mechanical durability, strong repellence to vapor, liquid, and solid matters, ice adhesion, heat-transfer efficiency, etc.Similarly, there are few analysis and discussion on the calculated results of these theoretical models.For example, it is pointed out that the covalent bonds are formed between coatings and various substrates.How the covalent bonds are formed and the relationship between the covalent bonds and the performance of the coating have not been properly discussed and analyzed.These theoretical analysis and discussion are very important for understanding this kind of coating.
2. In terms of the comparison of test results, especially the mechanical friction test, mechanical brushing test and substrate adhesion test, the author only conducted the comparision of the test results under different conditions for their own experiments, and did not compare these results reported by others.This will lead to a lack of rationality in comparison results.These comparision are important for the illustration of the coating performance.
Based on the above problems, I don't think this work can be published in Nature Communications.
Reviewer #3 (Remarks to the Author): The paper presents an interesting experimental study on the fabrication of durable superhydrophobic surfaces, with an extended characterization of durability in a variety of conditions.I think the study is interesting and worth publishing in this journal.However, I have some comments that the authors should address to improve the paper clarity.
1.I am not sure why the authors call the diatomite "shell" and silica "nanoseeds".Looking at the SEM in Fig. 1c, it seems that the silica nanoparticles are decorating the diatomite external surface.I thus do not really agree with the authors' claim: "Zoom-in SEM inspection reveals that the nanopores are impregnated with a large number of nanoseeds" (page 4).It looks to me that diatomite and silica are creating particles with dual scale roughness, which is known to be helpful with to enhance hydrophobicity.Can the authors provide better images to support their statement?2. What are the wetting properties of the epoxy matrix?I was not able to find the information in the paper or in the SI.In case the epoxy is hydrophilic, was it hydrophobized to ensure good adhesion between the matrix and the so-called cell?If not, how is good adhesion possible?3. Calling the roll of angle theta_r (Figure S12 to S19) is a bit misleading, as this is normally the symbol used to indicate the receding contact angle.I suggest changing the symbol.

Reviewer #1:
In this manuscript, the authors reported a cellular coating that imparts liquid-repellence, vaporimperviousness, and solid-shedding capabilities without the need for complicated structures and fabrication processes.The durability and strong resistance to accretion of different matters of our cellular coating are evidenced by strong anti-abrasion, enhanced anti-corrosion against saltwater over 1000 h, and maintaining dry in complicated phase change conditions.However, the manuscript lacked remarkable innovation because some similar component systems have been studied extensively (Advanced Functional Materials 2022, 32, 2113297.;Nature communications 2021, 12, 982;……).So, I suggest the authors further emphasize the highlights and distinguishing features of the manuscript.
I believe this manuscript could be considered after solving below additional concerns to meet the high impact requirements of Nature communications.

Response:
We thank the reviewer for reviewing and commenting on our manuscript, and allowing us to emphasize the novelty.The key innovation of our work lies in designing cells that consist of rigid microshells and releasable nanoseeds.The cells were mechanochemically controlled to impart the coating ultra-durability.Mechanically, cells act as a strong shield to protect the surface structures when the applied load is smaller than their critical fracture point.Whereas, at larger loads, the top cells can be broken and nanoseeds are instantaneously released by the shear force, featuring a shear-adaptive release, thus maintaining the water repellence.Chemically, we leveraged on the heterogeneous chemistry of the cells by fully salinizing the nanoseeds and partially salinizing the shells, which enables the cells to have a strong bonding strength with the matrix, meanwhile keeping a global superhydrophobicity.We have added these statements in the revised manuscript.Please kindly see Lines 40-50 in the manuscript.
The cellular design is distinct from those referred to by the reviewer.The work in Adv.Funct.
Mater., 2022, reported a strategy of introducing high-strength materials and adhesives to improve the mechanical durability of coatings.The dual-scale particles were used to create a structural hierarchy to enlarge the water repellency.Zhang's work (Nat. Commun. 2021, 12, 982) used a self-similar structure design to improve durability, that is, the water repellency was maintained by exposing underlying similar structures after abrasion.Although previous studies also used chemical components (e.g., silica) similar to our work, the mechanical shield or heterogeneous chemistry was not involved, a core innovation of our method.Moreover, compared with these strategies, our cellular design achieved 2-3 orders enhancement in the abrasion resistance, as characterized by the remarkable improvement of the wear coefficient, as we have presented in Fig. 3c in the manuscript.We have reviewed the mechanism of these reports in the revised manuscript and added them to the reference list.Please see Lines 30-33 and References 16 and 26.
Comment 1.It will be better if the authors could provide more detail preparation process of the superhydrophobic cell coating.
Response: Thanks for the valuable suggestion.We have added the detailed description of the preparation process in the revised manuscript.Please kindly see Lines 53-63.
Comment 2. Why did the authors use bisphenol A epoxy resin in the manuscript.In addition, as we all known, the organosilanes have so much variety.So why did the authors use octyltriethoxysilane rather than other silanes?Please provide more clearly reasons.
Response: Thank you for the comments.First, the criterion for choosing the matrix is the capability of adhering to a wide range of substrates.According to this, our cellular coatings could be prepared by various matrixes, including epoxy resin, polyurethane resin, polyacrylic acid resin, and ceramic matrix, which all obtained similar ultra-durability and multiphase repellence (Please see Lines 81-83 and Fig. 3 in the manuscript, and Figs.S8, S25 and S30 in the Supplementary information).In the current manuscript, epoxy resin was used to represent common matrix systems, which facilitates expanding the application of cellular coatings in different fields, i.e., maintaining the original coating systems in these fields.
Second, the criterion for choosing silanes is their reaction activity with -OH.A moderate activity allows us to control the density of the modified silane group on cells without forming sol-gels.We used octyltriethoxysilane as a representative because its terminal ethoxy groups are much more stable compared with some other groups, for example, the chlorine group of n-octadecyltrichlorosilane.The ethoxy groups need a certain amount of water to facilitate the hydrolysis reaction and form the hydroxyl groups at a relatively low speed.Then, the octyltriethoxysilane could graft onto the silica particles by dehydration condensation reaction to provide low surface energy.
Comment 3. In Equation 6, E1, and E2 are the elasticity modulus of cells, and matrix.How obtain the E1 and E2 values?Or how to test the two values?
Response: We thank the reviewer for the thoughtful comments.The elasticity modulus of cells and matrix was directly obtained by the nanoindentation tests.We have added the methods in the revised version.Please kindly see Section 4.2 in the revised Supplementary information.
Comment 4. The authors provided theoretical models for optimizing the coating strength in supporting information, so could you educe the relationship between the coating microstructure (e.g., particle and cell diameters, particle and cell distributions, and quantities) and the coating strength?I think the theoretical models have a deeper impact on the superhydrophobic coating.
Response: Thank you for the helpful suggestions.In the previous modeling, we have studied the influence of cell content on the coating strength.The cell distribution is one of the parameters that determines the heterogeneity of covalent bonds in the coating.However, since the cell distribution and heterogeneity of covalent bonds are both hard to characterize using the existing methods, we used a parameter C for correction.Please see Equation 2 in the manuscript (Lines 93-94).
Here, we further analyzed the influence of the cell size on coating strength as follows.The mechanical strength of the cellular coating with different cell diameters d (here, d is the average diameter characterized by laser particle size analyzer, Mastersizer 3000, UK) can also be predicted based on the Griffith-Irwin-Orowan theory, which can be expressed as the following equation: where E is the elasticity modulus of the coating, is the total plastic work before the coating breaking, and a is the crack length.The elasticity modulus of composite coatings can be represented by a generalized rule of the form as below: where E, E1, and E2 are the elasticity modulus of coating, cells, and matrix, respectively, and V, V1 and V2 are the volume fraction of coating, cells, and matrix, respectively.Then, V1 and V2 can be respectively expressed as follow: where d1, d2, d3 …dN are the diameter of each cell, h1, h2, h3 …hN are the height of each cell, and i1, i2, i3 …iN are the correlation coefficient between diameter and height, respectively.Therefore, Eq. ( 2) can be deduced as: where j1, j2, j3 …iN are the correlation coefficient between the diameter of each cell and the average diameter.As the initial crack is the interface between the cell and matrix, the crack length a is in direct proportion to the cell diameter d: By substituting Eq. ( 5) and ( 6) into Eq.( 1), the relationship between the coating fracture strength and cell diameter d can be expressed as: that is: where parameters P and Z are the contribution factors illustrating the reinforcement effect of the cell and the deterioration effect induced by the initial crack in the interface of the cell and matrix, respectively.In other words, when the cell diameter is relatively small, the reinforcement effect of the cell plays a dominant role.On the contrary, when the cell diameter is relatively large, the deterioration effect of the crack determinates the coating strength.
The fit of Eq. ( 8) with the experimental data points in Fig. R1, demonstrated a high consistency between the theoretical calculation and experimental results, suggesting the feasibility of the Griffith-Irwin-Orowan theory for predicting the mechanical strength of superhydrophobic coatings.
We have added the corresponding discussion and Fig. R1 to the revised version.Please see Lines 94-97 in the revised manuscript, and Fig. S10 and Supplementary Text 3 (Pages 14-15) in the revised

Reviewer #2:
This work propose a cellular coating design approach.The coating is mainly composed of micro-sized porous diatomite shell, releasable silica nanosphere seed and multipurpose matrix.Meanwhile, finite element (FE) modeling of physical shield, simulation of chemical bridge and theoretical models for optimizing the coating strength are also provided.The experimental test results show that the cellular coatings have strong mechanical durability, good superhydrophobicity, substrate adhesion and strong repellence to vapor, liquid, and solid matters.The approach of the coating design is new and more creative.This work provides a new feasible way for engineering ultra-durable coatings.However, some issues should be addressed after the review.
Response: We thank the reviewer for reading and commenting on our work and also appreciating the novelty and impact of our work.The comments are addressed point-by-point below.
Comment 1.Although the authors demonstrated the excellent performance of the coating through experimental tests, the necessary theoretical analysis and discussion were missing for each performance test result.Such as these results of the tensile fracture strength, the mechanical durability, strong repellence to vapor, liquid, and solid matters, ice adhesion, heat-transfer efficiency, etc.
Similarly, there are few analysis and discussion on the calculated results of these theoretical models.
For example, it is pointed out that the covalent bonds are formed between coatings and various substrates.How the covalent bonds are formed and the relationship between the covalent bonds and the performance of the coating have not been properly discussed and analyzed.These theoretical analysis and discussion are very important for understanding this kind of coating.

:
Thank you for the critical suggestions.First, in our previous manuscript, we have built models to analyze the relationship between coating parameters (e.g., cell content and covalent bond density) and the coating strength.Further, as suggested by Reviewer 1, we have analyzed the influence of cell size on the coating strength, please see the response to Comment 4 in Pages 3-5 of this response letter.All the theoretical modeling has been validated by the experiments.Please kindly see Fig. 1 in the manuscript and Fig. R1 in the response letter.The cellular design and theoretical modeling allow us to develop superhydrophobic coatings with strong mechanical strength and a compact and continuous bulk phase, which are the basis for achieving ultra-durability and functional performance.